The invention concerns a process for drying sewage sludges or sludges of similar consistency with which impurities are separated from vapors occurring during drying.
A method of this kind has become known in the art through DE 39 43 366 A1.
Environmental protection has been a central theme in public discussions in the industrialized nations during the last two decades. As a result thereof, a plurality of legal requirements have been created e.g. in the European Economic Community. Corresponding action has been taken by the state and community authorities and by the industrial sector as a result of which an improved sewage treatment, i.e. sewage processing or purification, has been achieved.
Beginning with the 1976 guidelines of the EEC concerning pollution due to drainage of certain dangerous materials into the waters of the EEC and up to the EEC Guide Lines of May 1991 concerning the treatment of community sewage and improvements in the general treatment of sewage, there has been a yearly increase in the overall amount of effort expended and the associated funding in order to effect improved purification of sewage from industrial installations and private households. As a logical consequence of sewage purification, recent years have seen an increase in the associated amount of sewage sludge. This will increase even further in the coming years. One has therefore searched for a solution to the problem of how this sewage sludge, an unavoidable waste product of sewage treatment, can be further processed, utilized or disposed of. Sludges having similar consistency occur in industrial installations e.g. in painting installations, washing installations of all kinds and the like.
Since the disposal of sewage sludge is becoming increasingly difficult and expensive due to the shortage of available dumping facilities, a mass and volume reduction in the amount of generated sewage sludge is necessary prior to disposal or further processing. The dumping of sewage sludge will be forbidden by law beginning in the year 2005. A reduction of volume simplifies transportation problems as well as the handling of the sewage sludge. This concerns both the directed treatment e.g. purification, further processing and, if appropriate, even the recycling of the sewage sludge.
Various methods are utilized either separately or collectively to reduce the mass and volume of the sewage sludge involving, for example, influencing evaporation of the moisture bound within the solid sludge components (drying), the decomposition of organic materials through removal of gases as well as the oxidation of organic substances through incineration.
DE 39 43 366 A1 proposes processing steps relating to the temperature of a fluidized bed, to the introduction of solids into the fluidized bed which have large amounts of material which can evaporate, to the removal of dried solids from the fluidized bed, as well as to the gaseous components of the evaporating material removed along with the vapor and other gaseous impurities.
An energy balancing of these processes of treatment and utilization of the sewage sludge takes into consideration the fact that energy must be expended in order to carry out the mechanical liquid removal processes and the thermal drying including processing of the vapours. In contrast thereto, energy is freed when the dried sewage sludge is subsequently incinerated in a power plant. If the sewage sludge is dried, and subsequently incinerated without prior decomposition thereof, the energy balance is compromised. However, if a decomposition process is carried out, a longer processing time disadvantageously results, since the decomposition processes require a certain amount of time. A self-sufficient operation with regard to energy is not possible, since it is always necessary to use more energy than is freed. In order to influence the energy balance during subsequent treatment in a positive, environmental manner, measures for improvement of the drying process are required.
It is therefore the purpose of the present invention to optimize the conventional thermal drying process in such a fashion that the energy utilized during operation of the installation is more efficiently applied for drying the sewage sludge to achieve as positive an energy balance as possible subsequent to incineration of the dried sewage sludge.
This purpose is achieved in accordance with the invention with a method for drying sewage sludge or sludges of similar consistency in which sewage sludge or sludges of similar consistency are processed in a contact drying device from which the vapors occurring during contact drying are introduced to at least one compressor in which the vapors are brought to a higher pressure and impurities are separated out of the vapors, and the purified and increased pressure vapors are utilized as a heating medium in the contact drying device.
A contact drying device is used for effective heating of the sewage sludge.